Reactions of complex ligands, part 92.: Tetrakis Fischer-type carbene complexes:: Synthesis, molecular structure, and [3+2+1]-benzannulation

Tetrakis{pentacarbonyl[(methyleneoxy)phenylcarbene]chromium}methane (3) and tetrakis( pentacarbonyl[(methyleneoxy)(2-methoxyphenyl)carbene] chromium)methane (4) have been synthesized from tetramethylammonium acylchromate precursors 1 and 2 and pentaerythritol by an acylation/alcoholysis sequence. The X-ray structure analysis of 3 established a distorted-tetrahedral. structure, whereas an averaged regular tetrahedral arrangement is indicated for 3 and 4 by NMR spectroscopy in solution. Upon reaction with 3-hexyne the phenyl complex 3 underwent complete benzannulation at all four carbene centers. The reaction was only moderately diastereoselective, and resulted-after in situ silylation-in at least seven inseparable stereoisomers of tetrakis-(CO)(3)Cr-coordinated tetrakis[1-((tert-butyldimethylsilyl)oxy)-2,3-diethyl-4-(methyleneoxy)naphthyl]methane. Oxidative demetalation gave an 80% yield of tetrakis[1-((tert-butyldimethylsilyl)oxy)-2,3-diethyl-4-(methyleneoxy)-naphthyl]methane (5). Benzannulation is blocked by increasing steric bulk of the arylcarbene ligand such as in the (2-methoxyphenyl)carbene complex analogue 4.